The late Carl Sagan might be stunned to see today’s astronomers questioning Earth’s “humdrum” status.
Some recent science articles make Earth look mighty special. At Discover Magazine, Nathanial Scharping offers the suggestion that “Earth may be a 1-in-700 quintillion kind of place.” That’s a couple of orders of magnitude lower odds than the number of stars estimated in the universe.
A new study suggests that there are around 700 quintillion planets in the universe, but only one like Earth. It’s a revelation that’s both beautiful and terrifying at the same time.
Astrophysicist Erik Zackrisson from Uppsala University came up with this number in a computer model, so it could be criticized on that basis. His paper, published on Cornell’s arXiv server in advance of expected publication in the Astrophysical Journal, assumes the big bang and models of stellar, galactic and planetary evolution. Even so, his conclusions mark a startling departure from typical cosmological speculations about the likelihood of habitable planets. Since there are so many stars and planets out there, the thinking has been since Carl Sagan’s days, life must be common.
But according to Zackrisson, most planets in the universe shouldn’t look like Earth. His model indicates that Earth’s existence presents a mild statistical anomaly in the multiplicity of planets. Most of the worlds predicted by his model exist in galaxies larger than the Milky Way and orbit stars with different compositions — an important factor in determining a planet’s characteristics. His research indicates that, from a purely statistical standpoint, Earth perhaps shouldn’t exist.
Zackrisson’s model will likely be debated and challenged. “Nevertheless,” reporter Scharping felt comfortable saying, “the researchers are confident in the broader implications of their model: Earth is more than your garden-variety planet.”
Bob Yirka took this subject up in PhysOrg. Zackrisson, he says, is one of a team of four that arrived at their conclusion. It appears that part of their thinking relies on the Fermi Paradox, which reasons, if advanced civilizations are out there, why haven’t they visited us yet?
If correct, the models suggest that Earth is much more unique than other models have been showing in the past few years. This is because it is assumed that if life began on other planets far earlier than on Earth, because it would be much older, it should have matured beyond what we have here on Earth to the point that it would be not only noticeable to us, but likely dominant. But because we have not seen any sign of other life, it appears likely that none is there, or is close enough to spot, which suggests that Earth actually is much more unique than other recent models have been suggesting. The model also suggested that most exoplanets likely exist in galaxies that are a lot bigger than the Milky Way, and orbit stars that are quite different from our sun. To date, space scientists have identified approximately 2,000 exoplanets, clearly a very small proportion of the total amount if the new model is to be viewed as accurate.
Because this conclusion contradicts the “Copernican Principle” that has achieved the status of dogma in modern cosmology (i.e., that Earth occupies no privileged position), it is likely to be vigorously debated.
Do astronomers know how planets form? Space.com posted excerpts of a dialogue between astronomers at the Kavli Roundtable on the subject. Our recent post on hot Jupiters (2/24/16) described the exasperation those caused to planetary formation models. In the Kavli dialogue, we find Ruth Murray-Clay admitting that accretion of any planet is a problem. Hoping that a newly-observed exoplanet will shed light on accretion, she states, “It’s very difficult to model that process theoretically, partly because we just don’t know how accretion happens in young disks and onto planets.” The article also confesses that core accretion models are problematic accounting for gas giants in any location, whether in tight orbits near their host stars (hot Jupiters) or in distant orbits. A figure caption says, “Overall, many questions remain about how, where and when planets arise around stars.”
The panel concluded with expressions of confidence in the Copernican Principle, attributing our lack of knowledge about planet formation to the fact that exoplanet science is in its infancy. Nevertheless, the uniqueness of Earth cannot be ruled out.
A lot of us are driven by questions about Earth-like planets, of course. How common they are? How detectable might they be? Can we see them? Planet formation theorists like Ruth are trying to understand whether planets like Earth ultimately are a common thing or a rare thing, and whether our Solar System is unique or whether there’s a whole bunch of others out there like it.
Speculations about alien habitats have not slowed down, of course. Steven J. Dick has a new book on the subject with the usual talking points (see Astrobiology Magazine review). Space.com expanded the search space to “exomoons” around exoplanets, complete with references to Hollywood movies about Endor’s Ewoks and Pandora’s Na’vi. One must remember, however, that hosting life requires more than an appropriate-sized planet in a suitable location. A Harvard press release points out that Earth-like planets will need to have Earth-like interiors, too. And Becky Oskin at Live Science points out that alien life would be vulnerable to greenhouse effects on their worlds (think Venus). That kind of warming is as powerful as proximity to the sun. Given an increase in carbon dioxide, “A planet like Earth will eventually change to a very warm climate, and it will occur relatively abruptly,” one researcher said.
Then there are other protective mechanisms required, like a long-lived magnetic field (see Nature Communications) and Van Allen Belts (PhysOrg). Too many bombardments can also ruin an alien’s whole day (PhysOrg). Another factor not often considered is how microbial life contributes to the habitability of a planet (Evolution News & Views). This would create a chicken-and-egg dilemma; to be habitable, a planet might need to be inhabited (6/06/14).
National Geographic listed five cosmic mysteries that scientists still hope to solve. Three of the five deal with habitability and inhabitants: #3, “Are We Alone?”, #4 “Where Is Everyone?” and #5, “Is Our Universe Alone?” That last one leaves science behind, Michael Greshko admits:
Just as Earth seems improbably friendly to life, some fundamental aspects of the universe sit at suspiciously convenient values—a pattern called the naturalness problem. Increase dark energy’s value slightly, for instance, and the early universe would have expanded too quickly for galaxies to stick together.
Yet the universe’s overwhelming size all but guarantees that Earth-like planets exist, as a matter of probability. Along this line of thinking, some physicists argue that, like Earth among planets, our universe is one of innumerably many—but that ours happens to have the conditions that allow us to exist. Otherwise, we wouldn’t be here to study and write about it.
Proponents of this multiverse model claim that it neatly explains our universe’s habitability, but many scientists find it irritatingly circular in its reasoning. Proving or disproving such a sweeping statement verges on the philosophical, and ultimately will have to grapple with how common universes like ours are in comparison to other varieties.
For now, it seems appropriate to infer two conclusions from these articles: (1) planet formation is not well understood, and (2) the uniqueness of Earth cannot be ruled out scientifically. Fermi’s Paradox still calls out: if aliens are common, where are they?
Carl Sagan influenced a generation with his vision of Earth as a lonely speck around a humdrum star in a universe filled with aliens. That worldview is not holding up to increased observations. Astronomers cannot explain planet formation, star formation, or galaxy formation, and for sure they are clueless about the origin of life. Earth is still the only known place where any life can be found. There is still ample evidence to support the conviction that intelligent life on Earth is not only special, it is unique—even in a universe of quintillions of stars. Don’t let anyone browbeat you into believing that science has proved otherwise.
Exercise: Why would the conclusion of Earth’s uniqueness be “both beautiful and terrifying”? Think about that. To whom would it be terrifying? Why?